Subset of MERGE queries lead to setting unexpected property values

What Happened

An issue was introduced in the Neo4j 2026.02 release where MERGE queries that referenced the same property on both the left and right sides of an ON MATCH SET or ON CREATE SET clause could potentially delete that property from the node or set it to an invalid value during query execution.

This behaviour was observed specifically when the node being matched had at least one property uniqueness constraint.

How the service was affected

A change in the Neo4j 2026.02 release introduced a potential risk of writes failing or invalid data being returned for queries using MERGE together with ON MATCH.

This issue affected instances across Aura tiers and required immediate investigation by Neo4j Engineering. The team identified the root cause and deployed a fix in version 2026.02.1.

What are we doing now

The following proactive measures have been implemented to reduce the likelihood of similar incidents:

• We have strengthened test coverage for ON CREATE and ON MATCH clauses, particularly those involving more complex expressions.
• We are investigating additional safeguards to improve our ability to control MergeInto/MergeUnique behaviour more flexibly, as well as potential rollback capabilities to support recovery in future incidents